This invention relates to inverter circuits for converting DC voltage to AC voltage, and especially to inverters of the type that use a center-tapped load. More particularly, the invention relates to a means for varying the waveform generated by a center-tapped-load type inverter in order to permit control of the effective AC voltage level produced.
Standby power equipment, often including batteries, is frequently used in systems having loads that must be quickly energized during a failure of commercial power. Such systems are referred to as "uninterruptible power systems." Typical applications where such standby power sources are required are in communication systems (telecommunications--especially in switching stations), computer systems such as in the case of hospital records wherein patient information in a memory must not be lost, and process control systems.
In many of these applications, it is important that the standby power source be put on-line very quickly in the event of a primary power failure. If it were not, computer memory could be lost or communications could be interrupted, each with disastrous consequences.
Accordingly, standby batteries with DC to AC inverters are often used to quickly place a desired voltage on-line in the event of a commercial power failure. In some cases, an AC voltage from the secondary winding of an inverter transformer is used directly, and in other cases the resulting AC voltage is rectified to a DC voltage at the desired level.
The design of suitable circuits for inverters has resulted in two basic types, the so-called "bridge" type and the "center-tapped transformer" type. The bridge type, such as those shown in U.S. Pat. Nos. 3,422,342 and 4,196,468, has the advantage of having a variable AC voltage output. The variation of the voltage waveform is accomplished by adjusting the phase angle between the two branches of the bridge.
This type of inverter has two major disadvantages, however, in the more common applications. First, two sets (pairs) of commutating capacitors are needed and, second, all power must flow through two gate-controlled rectifiers (SCR's), a condition that seriously limits the efficiency of the inverter.
The second basic type of inverter is the center-tapped-load type, such as those disclosed in the following U.S. Pat. Nos.: 3,317,816; 3,407,349; 3,424,973; 3,702,432; 3,781,643; 4,161,773; 4,274,137.
This type of inverter circuit has greater efficiency than the so-called "bridge" type, especially at low voltage levels, but prior art designs do not permit variation of the voltage waveform produced. The voltage produced across the secondary windings of the transformer in this type of inverter circuit is a square wave, and thus the waveform has no zero voltage intervals that could be adjusted to permit variation.
In these circuits, one terminal of the DC source is connected to an inductor and, in turn, through a pair of parallel, gate-controlled rectifiers (SCR's), to the opposite ends of a transformer primary winding. A center tap of the primary winding is connected to the other terminal of the DC source and a commutating capacitor is connected across the transformer primary winding. A trigger voltage signal is generated for alternately gating the rectifiers to produce the square wave across the primary winding of the transformer.
As indicated above, this circuit does not permit varying of the voltage level of the square wave generated across the secondary winding of the transformer because either one or the other of the SCR's must be conducting current at all times. Accordingly, the output voltage has only two states--positive or negative. In order to produce a variable waveform, a third state is required--namely, a condition of zero voltage across the secondary winding of the transformer.
The circuit of the present invention achieves the desired capabilities described above, and affords other features and advantages heretofore not obtainable.